[libcxx] Use Fuchsia-native monotonic clock for std::chrono::steady_clock

[mcgrathr]

Summary

Use the zx_clock_get_monotonic system call directly rather than
going through the POSIX clock_gettime function. The libc function
is a trivial wrapper around the system call, and is not a standard C
function. Avoiding it reduces the Fuchsia libc ABI surface that
libc++ depends on.

[phosek]

LGTM

libcxx/src/chrono.cpp

273–284

The rest of this file doesn't seem to be using this indentation.

[mcgrathr]

libcxx/src/chrono.cpp

273–284

AFAICT it's not consistent within the file. There are spaces after # in the some of the blocks at the top but not everywhere else.
The formatting in this change is the result of arc diff doing its idea of lint checks, which AIUI is clang-format on the lines mentioned in the diff.

[mcgrathr]

Ping. This is very similar to the other change I landed recently. It needs approval from a libc++ maintainer.
Thanks.

libcxx/src/chrono.cpp

 #  define WIN32_LEAN_AND_MEAN
 #  define VC_EXTRA_LEAN
 #  include <windows.h>
 #  if _WIN32_WINNT >= _WIN32_WINNT_WIN8
 #    include <winapifamily.h>
 #  endif
 #endif // defined(_LIBCPP_WIN32API)
 
+#if defined(__Fuchsia__)
+#  include <zircon/syscalls.h>
+#endif
+
 #if __has_include(<mach/mach_time.h>)
 # include <mach/mach_time.h>
 #endif
 
 #if defined(__ELF__) && defined(_LIBCPP_LINK_RT_LIB)
 #  pragma comment(lib, "rt")
 #endif
 

 static steady_clock::time_point __libcpp_steady_clock_now() {
   struct timespec64 ts;
   if (0 != gettimeofdayMonotonic(&ts))
     __throw_system_error(errno, "failed to obtain time of day");
 
   return steady_clock::time_point(seconds(ts.tv_sec) + nanoseconds(ts.tv_nsec));
 }
 
-#elif defined(CLOCK_MONOTONIC)
+#  elif defined(__Fuchsia__)
+
+static steady_clock::time_point __libcpp_steady_clock_now() noexcept {
+  // Implicitly link against the vDSO system call ABI without
+  // requiring the final link to specify -lzircon explicitly when
+  // statically linking libc++.
+#    pragma comment(lib, "zircon")
+
+  return steady_clock::time_point(nanoseconds(_zx_clock_get_monotonic()));
+}
+
+#  elif defined(CLOCK_MONOTONIC)
 
 static steady_clock::time_point __libcpp_steady_clock_now() {
     struct timespec tp;
     if (0 != clock_gettime(CLOCK_MONOTONIC, &tp))
         __throw_system_error(errno, "clock_gettime(CLOCK_MONOTONIC) failed");
     return steady_clock::time_point(seconds(tp.tv_sec) + nanoseconds(tp.tv_nsec));
 }
 
-#else
-#   error "Monotonic clock not implemented on this platform"
-#endif
+#  else
+#    error "Monotonic clock not implemented on this platform"
+#  endif
 
 const bool steady_clock::is_steady;
 
 steady_clock::time_point
 steady_clock::now() noexcept
 {
     return __libcpp_steady_clock_now();
 }
 
 #endif // !_LIBCPP_HAS_NO_MONOTONIC_CLOCK
 
 }
 
 _LIBCPP_END_NAMESPACE_STD